The shock absorption of an athletic shoe depends primarily upon the material used for the sole of that shoe. Moreover, the weight of such a shoe is primarily effected by the weight of the sole as that is usually the heaviest component of the shoe. There has been an evolution in the sports shoe industry to obtain a lighter and more shock absorbent athletic shoe.
Light weight elastomeric materials such as polyurethane are now used to make the soles but such materials each have a certain density and elasticity and hence can only approximate the optimum characteristics for the weight of the user of that shoe. It is important to keep in mind that as the weight of each person wearing athletic shoes varies, this translates into a substantially different downward force, which force is concentrated in the sole of the shoe. Present shoes cannot be "tuned" or "tailored" to the particular degree of activity the user wishes to subject his feet to or the weight of the user and therefore shoes are made to a common denominator.
In order to prevent damage to the foot, the manufacturers have worked out combinations of two or more materials having different elasticities and have molded shoe soles out of these materials. Of course, when a sole is made up of two or more materials having different characteristics, the juxtaposition of these materials is unchangeable and therefore that particular shoe will always have the same characteristics. Again, such a shoe cannot be altered once manufactured and therefore cannot be tailored to the needs of the wearer.
The industry has produced shoes with air cushions and this has been an advantage because the air cushions act as a shock absorber. When the localized upward force is directed to a portion of the sole, for example if a runner were to step on a rock, the upward force vectors are absorbed by the air cushion and more evenly distributed throughout the air cushion cavity to a larger surface of the foot of the wearer. Normally, the air cushion is filled with air and is inflated at the factory to a predetermined pressure and has no means of adjusting that pressure. See for example U.S. Pat. No. 2,080,469. Moreover, if the air cushion were punctured, there is no vertical support to the air cushion once it is punctured and therefore the cushion deflates and the shoe becomes useless. Manufacturers have made some air tubes in shoes with a tire valve in them, see U.S. Pat. No. 3,005,272, but this proves an inconvenience having to go to a gas station to inflate the shoe, and again if there is a puncture, the air cushion collapses. Another development was to fill the cushion cavity with various types of specialized gases and encapsulate the cushion within foam material, see U.S. Pat. No. 4,219,945 but again, when punctured, the air cushion loses its vertical support. Another approach to shock absorbing air cushions was to develop a two stage shock absorbing shoe sole as see in U.S. Pat. No. 4,263,728 and again, if punctured, the shoe lost its value.
The prior development of air cushion shoes failed to provide an air cushion that would have vertical support even if it was punctured so that the shoe would still have use if punctured, which many times happens. Moreover, the air cushions of the prior developments either were factory sealed to a given pressure and had no means whereby pressure adjustment could be made, or had metal tire valves or other types of air valves in them. The metal tire valves were difficult because they had to be embedded within the plan profile of the sole and hence affected the performance of the shoe and could be usually felt by the wearer. The other valves were usually one way valves, and could not or certainly the developers did not teach that the air in the air cushion could be adjusted to the wearer's needs and hence the shoe could not be tailored to the person wearing the shoe. None of the prior developments taught an air valve which had an automatic bleed off means so that if the pressure in the air cushion cavity got to a predetermined level, the pressure could automatically bleed through the air valve thereby further absorbing the shock which caused the pressure increase.
The method of manufacturing an air cushion for prior development include more than two processes. The air cushion must be made in two pieces and sealed together. That caused higher cost and lost time.